I tend to always give NKG a hard time but ironically after selling some stuff on ebay I can see two main reasons for the slowly climbing prices.

1) People keep buying, even though the prices keep rising. Hard to argue with market economics. You can always wait and find something cheaper but most people don't like to wait.
2) It is really annoying to watch material be bought by a re-seller and sold again at even higher prices. Especially play condition material you're just trying to get in the hands of someone who wants to read it. If you try to sell shrink at any reasonable price a re-seller buys and gouges the price way up.

From that perspective I can cut them some slack but I still don't buy at their pricing level, I personally just wait. And the other antics on rare and high priced items are still unacceptable. lol